A $7 million five-year proposed contract headed to the City Council on Tuesday would install ISL G Near-Zero 0.02 NOx emission engines on 60 buses in the Big Blue Bus (BBB) system, a report to the council said.

BBB’s current fleet includes 200 buses.

Nitrogen Oxides are a family of "poisonous, highly reactive gases" that form when fuel is burned at high temperatures, according to the U.S. Environmental Protection Agency (EPA).